AIR PERMEABILITY The air permeability of a fabric is the volume of air measured in cubic centimeters passed per second through 1cm2 of the fabric at a

pressure of 1cm of water. AIR RESISTANCE

The air resistance of a fabric is the time in seconds for 1cm3 of a air to pass through 1cm2 of the fabric under a pressure head of 1cm of water.

AIR POROSITY The porosity of a fabric is a ratio of airspace to the total volume of the fabric expressed as a percentage

MEAS URE MENT OF AIR PE RMEABILITY

The Shirely air permeability apparatus is illustrated in fig. 7.12. Air at 20± 2OC and 65 ±2 % R.H. is drawn from the laboratory through the test specimen S by means of a suction pump A, the rate of flow being controlled by means of the by pass valve B and the series valve C. The rate of flow is adjusted until the required pressure drop across the fabric is indicated on a draught gauge D, graduated from 0 to 25mm water head.For fabrics of high resistance, the rate of flow of air through the specimen is inadequate for proper operation of the suction pump; this is over come by opening the by-pass valveB .Fine control is obtained by adjusting valve C.E is reservoir which smoothes out any disturbance due to the varying velocities of the streams of air drawn through the various paths by the pumpWhen the required pressure drop,which is normally 1cm of water, is attained and the indicator of the draught gauge is steady, the rate of flow of air is read off one of the four Rotameter R, selected according to the permeability of the test specimen The rotameters are calibrated, at 20OC and 760 mm of mercury, to indicate air flow in cm3 /s.

Rotameter cover the following range : R1 0.05 - 0.5 , R2 0.5 - 3.5, R3 3 - 35 and R4 30 - 350. Five circular specimens are clamped in the holder each with a test area of 508mm2 (25.4mm diameter) and the mean air flow per second is calculated from the five results and by dividing by 5.07 we obtain the air permeability of the fabric in Cubic Centimeters per Seconds at 1 cm head of water.Alternatively, 5.07 may be divided by mean flow; this gives air resistance of the fabric in seconds per cubic centimeters per square centimeter under a pressure of 1 cm of waterEFFECT ON FABRIC PROPERTIES

Permeability and Cloth Cover :- One would expect that the more open structure of the fabric the greater the air permeability, but this relationship is not always simple

one. For example, using given yarns the ends and picks per inch may be varied and the air permeability found to follow the variation in

cloth cover more or less as expected. However, it is possible to arrange a series of fabrics built from yarns of different counts with the yarn spacing adjusted to give each

fabric the same cover and to find large differences in their air permeability. Yarn Twist is also important in that as twist increases, the circularity and density of the yarn increases in a fabric.T his reduces the

yarn diameter and the fabric cover which increases the air permeability. Yarn crimp and weave influences the shape and area of the interstices between the yarns and may permit yarns to extend easily. Such yarn extension would open up the fabric, increase the free area and increases the air permeability. Hot calendaring can be used to

flatten yarns which reduces air permeabilityMeasurement of Thermal Conductivity:

Practical methods of test for thermal conductivity measure the total heat transmitted by both mechanisms i.e. by conduction through the fibre and the entrapped air and by radiation.

The insulation value of a fabric is measured by its thermal resistance which is the reciprocal of thermal conductivity (transmittance) and it is defined as the ratio of the temperature difference between the two faces of the fabric to the rate of flow of heat per unit area normal to the faces.

It is necessary to know the rate of heat flow through a fabric to measure its thermal resistance. In practice it is difficult as a heater dissipates its heat in all directions. Two different methods are in use to overcome this problem:

One is to compare thermal resistance of the sample with that of a known standard and Other is to eliminate any loss in heat other than that which passes through the fabric being tested

It is important that any measurement of thermal resistance are made at temperatures close to those that are likely to be encountered in use as the thermal conductivity of materials varies with the temperatures.Thermal Conductivity Measurement -TOG METER

The togmeter avoids the problem of measuring heat flow by placing a material of known thermal resistance in series with the material under test so that the heat flow is the same through both materials.

The thermal resistance of the test fabric can then be calculated by comparing the temperature drop across it with the temperature drop across the standard material.

Apparatus The togmeter consists of a thermostatically controlled heating plate which is covered with a layer of insulating board of known thermal resistance. The temperature is measured at both faces of this standard. The heater is adjusted so that the temperature of the upper face of the standard is at skin temperature (31-350C). A small airflow is maintained over the apparatus.

There are two methods of test that can be used with the togmeter. TOG METER:

• Two plate method.In this method the specimen under test is placedbetween the heated lower plate and an insulated top plate as shown in Fig.The top plate has a low mass so that

It does not compress the fabric. The temperature is measured at the heater (T1), between the standard and the test fabric (T2) and between the fabric and the top plate (T 3).

• Single plate method In this method the specimen under test is placed on the heated lower plate as above but it is left uncovered as shown in Fig., the top plate being used to measure

the air temperature(T3).• The air above the test specimen has a considerable thermal resistance itself so that the method is in fact measuring the sum of the

specimenThermal Conductivity Measurement:TO DETERMINE THE AIR RESISTANCE: The heater and the fan are switched on and the apparatus is allowed to reach Thermal equilibrium with no specimen present. The top plate is placed underneath the apparatus shielded from radiation by a foil-covered plate,in order to measure the air temperature.T he temperature should remain steady at each thermocouple for 30 mins. It may take some time for an equilibrium to be reached.Thermal resistance of air:R air = R stand x T2-T3/T1-T2R sample = R stand x T2-T3/T1-T2 – R airThermal Conductivity Measurement:- Guarded Hot PlateThe guarded hotplate is used to measure thermal transmittance which is the reciprocal of the thermal resistance. The apparatus consists of a heated test plate surrounded by a guard ring and with a bottom plate underneath as shown in Fig. With the test fabric in place the apparatus is allowed to reach equilibrium before any readings are taken. This may take some time with thick specimens.The amount of heat passing through the sample in watts per square meter is measured from the power consumption of the test plate heater.The temperature of the test plate and the air 500mm above the test plate are measured.

• All three plates consist of heating elements sandwiched between aluminum sheets. All the lates are maintained at the same constant temperature in the range of human skin temperature (33- 360C). The guard ring and bottom plate, which are maintained at the same temperature as the test plate, ensure that no heat is lost apart from that which passes upwards through the fabric under test. The whole apparatus is covered by a hood to give still air conditions around the specimen.

• The whole of the surroundings of the apparatus is maintained at fixed conditions between 4.5 and 21.10C and 20 and 80% RH, the exact conditions being specified as part of the test.

• The measured thermal transmittance consists of the thermal transmittance of the fabric plus the thermal transmittance of the air layer above the fabric which is not negligible.Therefore the test is repeated without any fabric sample present to give the bare plate transmittance. The transmittance of the air layer above the plate is assumed to be the same as that of the air layer above the sample.

Combined transmittance of specimen and air UvU1=p/A x (Tp-Ta) X W/m2 KWhere p= power loss from test plate(W). A = area of test plate (m2) , Tp = test plate temperature (C), Ta = air temperature ( C ).The transmittance of the air layer above the plate is assumed to be the same as that of the air layer above the sample. The bare plate transmittance Ubp is similarly calculated and then the intrinsic transmittance of the fabric alone,U2, is calculated from the following equation: 1/U2 - 1/U1 – 1/Ubp

Measurement of Water Vapour Permeabilit: The water vapour permeability of fabrics is an important property for those used in clothing systems intended to be worn during

vigorous activity. The human body cools itself by sweat production and evaporation during periods of high activity. The clothing must be able to remove this moisture in order to maintain comfort. This is an important factor in cold environments. The main materials of interest are those fabrics that incorporate a polymer layer that makes the fabric waterproof but which still allows

some water vapour to pass through. There are two main types of these materials:

1. those that contain pores through which the moisture vapour can pass and2. those containing a continuous layer of hydrophilic polymer

• The mechanism of water vapour transmission through the second type is quite different from that of the first type. • In particular the rate of diffusion through the hydrophilic polymer layer is dependent on the concentration of water vapour in the

layer.• The higher the concentration, the higher the rate of transfer. • In the materials where transmission is via pores the rate is independent of water vapour concentration. • This has a bearing on the results obtained from the different methods of testing water vapour permeability.

MOISTURE TRANSPORT:In order to keep the wearer dry and hence comfortable, Clothing that is worn during vigorous activity, such as sports clothing, has to be able to deal with the perspiration produced by such activity. There are two main properties of clothing, that affect the handling of moisture.

• Firstly there is the ease with which clothing allows the perspiration to be evaporated from the skin surface during the activity.

• Secondly after the activity has ceased, there is a need for the moisture that is contained in the clothing layer next to the skin to dry out quickly. This ensures that the wearer does not lose heat unnecessarily through having a wet skin.

Moisture is transmitted through fabrics in two ways: 1. By diffusion of water vapour through the fabric.This appears to be independent of fibre type but is governed by the fabric structure. 2. By the wicking of liquid water away from the skin using the mechanism of capillary transport.The ability of a fabric to do this is dependent on the surface properties of the constituent fibres and their total surface area. The size and number of the capillary paths through the fabric are also very important. Also the rate of wicking may be different along the warp (wale) direction than along the weft (course) direction. The points to consider in Moisture transport are :

• Wetting • Wicking ( Longitudinal Wicking) • Wicking Test • Transverse Wicking

WETTING • For wicking to take place the fibre has first to be wet by the liquid. • In fact it is the balance of forces involved in wetting the fibre surface that drives the wicking process. When a fibre is wetted by a

liquid the existing fibre-air interface is displaced by a new fibre-liquid interface. • The forces involved in the equilibrium that exists when a liquid is in contact with a solid and a vapour at the same time are given by

the following equation: • The contact angle is defined as the angle between the solid surface and the tangent to the water surface as it approaches the solid; the

angle is shown as inFig.• The angle is determined by the three interfacial tensions: if γSV is larger thanγSL then cosθ is positive and the contact angle must be

between 0° and 90°. IfγSV is smaller than γSL then the contact angle must be between 90° and 180°. • A high contact angle for water with the surface means that water will run off it, a low contact angle means that water will wet the

material. Acontact angle of less than 90° also means that water will wick into the material by capillary action. A contact angle of 90° or more means thatwater will not rise by capillary action.

WICKING• In the absence of external forces the transport of liquids into fibrous assemblies is driven by capillary forces that arise from the wetting

of the fibre surfaces described above. • If the liquid does not wet the fibres it will not wick into the fibrous assembly. In the case of contact angles above 90°, liquid in a

capillary is depressed below the surface instead of rising above it.

• In order for the wicking process to take place spontaneously,the balance of energy has to be such that energy is gained as the liquid advances into the material, therefore γSV must be greater

than γSL